Modulators are used in radio communication devices when, for example, frequency conversion is needed, such as up- and down-converting between baseband and radio carrier frequencies. Frequency conversion is performed by multiplying the input signal with a tone that has energy at the wanted frequency. A common implementation to achieve frequency conversion is to use a passive mixer and switch the polarity of the signal at the rate of the carrier frequency.
A passive mixer is an electronic circuit device configured to perform frequency conversion using one or more passive devices, such as diodes, versus active devices such as amplifiers, whether in the form of operational amplifiers (op-amps), in one example, or one or more transistors configured to perform the amplification/mixing function. Passive mixers use less power, are simpler in configuration and generally, with the proper selection of passive devices, have good (i.e., low) intermodulation distortion. Passive mixers, because of their “non-active” nature, cannot and do not add any gain to the output signal, and hence if signal level is important or critical an additional amplifier (or more) maybe needed to get the signal to the desired level. Or, in other words, by definition passive devices will suffer conversion loss. To avoid excessive noise contribution, the switching transitions between conducting and non-conducting modes are made short. During the transitions, the output voltage level is related to the level of the multiplying tone. As the amplitude of the multiplying tone is further increased or decreased, however, the gain saturates, which makes the tone multiplication with the input signal mimic multiplication of the input signal with a square wave. The Fourier expansion of a square wave shows that the multiplication tone also carries strong odd-harmonic components, e.g., at three, five, seven, and so on, times the carrier frequency.
Thus, performing frequency conversion by multiplication of the input signal with what is essentially a square wave introduces odd harmonics. For example, in a transmitter consider that the baseband signal, represented as fBB, is up-converted to 1×Fc+fBB, where Fc is the carrier frequency. However, due to the multiplication with a square wave there is also an unwanted frequency up-conversion of a third order harmonic to 3×Fc−fBB. In a succeeding stage of the transmitter, such as a power amplifier stage that operates close to compression due to the efficiency benefits of such an operation, these harmonic tones inter-modulate and create out-of-band energy at 1×Fc−3×fBB, potentially violating standards related to noise requirements, such as those promulgated by 3GPP for LTE. Therefore, filtering the up-converted signal after the mixer and before amplification by, e.g., a nonlinear amplifier, can be important. In particular, such filtering can be important for the third order harmonic as the third order nonlinearity of a typical CMOS amplifier is larger than its fifth order nonlinearity.
To achieve lower third order harmonic levels, existing solutions propose multiplication of several different tones, each with a different phase, to mimic multiplication of the input signal to be converted with a sinusoidal wave instead of a square wave. A major drawback to this solution is the complexity and current consumption (i.e., power) when generating these phases. The power consumption of the circuits generating the multiphase tones is directly proportional with frequency, making the topology unsuitable for high frequency operation where low power consumption is needed. Devices wherein such considerations could be important include, among others, handheld cellular phones, and other transmitter devices with up-converters that operate in voltage mode wherein suppression of odd-order harmonics could be of importance.
Another existing solution is the use of an active mixer, and more particularly an active current-mode mixer, rather than a passive mixer to perform the frequency conversion. Voltage mode and current mode are two regulating conditions that control the output of a source of a signal. A voltage source can provide a constant or changeable output voltage as current is drawn from 0 to the full rated current specification of the signal generator. In these applications, while the signal generator runs in voltage mode, it maintains the output voltage while providing the required current to the load. A signal generator is generally modeled as providing a low output impedance when operating in voltage mode. Current mode works in a similar fashion, except it limits and regulates the output current of the signal generator to the desired level. When the signal generator runs in current mode, the signal generator provides a constant current into a variety of load voltage conditions including a short circuit. A signal generator operating in current mode is generally modeled as providing a very high output impedance. Note that in both cases the voltage of the signal generator will change (because it is a modulating signal) but in the voltage mode case, the current level will be variable, while in the current mode case, the current is substantially more fixed. As discussed in greater detail below, current mode devices are usually used in high power, i.e., high frequency, transmitters and receivers.
A generalization of a simple, active current mode mixer 100 is shown in FIG. 1A. The term “simple” refers to number of inputs and configuration of the mixer. Mixers can be complex, meaning it has both and an I and Q input, and therefore an I and Q local oscillator signal, known to be 90° out of phase with each other. A simple, mixer, on the other hand, is not complex, meaning it does not have I and Q inputs, and only one LO input signal. However, as known to those of skill in the art, because many circuits now transmit data using differential or complementary type signals (i.e., a “+” signal and an opposite polarity “−” signal, for purposes of noise immunity, the LO's will also need to be provided in terms of a “+” and “−” configuration, meaning of opposite polarity). Therein, the active current mode mixer circuit 100 includes baseband (BB) filter 104, which not only provides a baseband signal to the mixer, but also filters it before being fed into a buffer or common current source stage 105. Following buffer or common current source stage 105 is active mixer 106, which performs the up-conversion to RF frequencies. The output of active mixer 106 can be filtered with an RC or LC network, generally represented by harmonics filter 108, due to the high output impedance of the active mixer 106. The filtered RF signal can then be amplified by power amplifier 110 and coupled to antenna 112 for transmission. However, the benefits of the passive, voltage mode sampling mixer, especially good or low intermodulation distortion, are lost if active mixer 106 is used in the transmit chain.
Those of skill in the art can appreciate that the harmonics could be filtered in a passive, voltage mode sampling mixer circuit 120 as generally shown in FIG. 1B between mixer 124 and power amplifier 128, as was discussed above with respect to the active, current mode mixer assembly 100. Therein, elements 122-130 perform generally the same functions as elements 104-112 (those are similarly named), respectively, but without the signal being driven by a current source (i.e., 105), i.e., a passive mixing circuit. However, attempting to filter the output of passive voltage mode sampling mixer 124 with harmonics filter 126 yields somewhat poor results because the required low source impedance, which is up-converted from baseband to the carrier frequency, lowers the Q value (or Q-factor) of harmonics filter 126, which can be typically provided as an LC circuit. A low Q-factor means a higher bandwidth, generally allowing more noise to pass through harmonics filter 126 and then to power amplifier 128. Increasing the source impedance, for all frequencies, degrades the conversion gain.
Another option is to filter the signal after power amplifier 146 as is done in passive, voltage mode sampling mixer assembly 140 shown generally in FIG. 1C. In FIG. 1C, elements 142-150 correspond respectively to similarly labeled (i.e., named) elements 122-130 of FIG. 1B. By filtering the signal after power amplifier 146 instead of between mixer 144 and power amplifier 146, distortion created in the trans-conductance stage of power amplifier 146 is not filtered by harmonics filter 148. Instead, only distortion due to voltage swing at the output of power amplifier 146 is filtered by harmonics filter 148, and it therefore can operate much better than in the alternative scenario. The former distortion, the trans-conductance stage output of the amplifier, is at a frequency location that is very close to the signal frequency which makes filtering it very difficult. The latter distortion, that which is due to the voltage swing, is located at a frequency that is more conducive to filtering in the afore-described manner.
Accordingly, it would be desirable to provide methods, devices and systems which address these, and other, challenges.